(1) Field of the Invention
The present invention relates to a semiconductor laser device applicable to a blue-violet semiconductor laser device used for, for example, a light source for writing and reading of a high density optical disc, and a manufacturing method thereof.
(2) Description of the Related Art
A III-V group compound semiconductor laser device has been widely used as a light source for reading and writing of an optical disc such as a Compact Disk (CD) and a Digital Versatile Disk (DVD). As the optical disc becomes denser, a wavelength of the light source needs to be shortened. Therefore, the wavelength of 780 nm in an infrared region has been shifted to that of 650 nm in a read region. In order to realize further densification, a light source having a short wavelength is necessary. Here, a GaN III-V group nitride semiconductor (indicated as InGaAlN) is expected as a material to realize the above. A blue-violet laser device with several 10 mW output has already been commercially manufactured. For the realization of a next generation high-density optical disc system (Blu-ray Disc), a research and development has been actively pursued. In the optical disc system, it is a necessity for the high-speed writing operation to increase the outputs from the light source. Therefore, a research and development has also been actively conducted for realizing a high-output GaN semiconductor laser device that is made of the GaN III-V group nitride semiconductor.
In general, the semiconductor laser device used for the optical disc system has a striped waveguide in which light is confined and which is positioned closely to an active layer that generates light emission, and a resonator mirror formed by cleavage. As factors of preventing the semiconductor laser device from increasing outputs, the following problems are assumed: an end-face destruction; a kink caused by hole burning (observed in current-light output characteristics); an output saturation by a heat generation; and the like. Thus, the output of the conventional infrared/red semiconductor laser device has been increased by solving these problems. On the other hand, in the present GaN blue-violet semiconductor laser device that is expected the use for the next generation, a development of a waveguide structure in order to raise the kink level is mainly the most difficult problem rather than the problems of the end-face destruction and the heat generation. Therefore, the development for increasing outputs has been pursued by designing a dimension of the waveguide structure and controlling the structure.
The following describes a laser structure of a ever-reported high-output infrared semiconductor laser device and an example of the laser structure of the high-output infrared semiconductor laser device applied to the GaN blue-violet semiconductor laser device.
FIG. 1 is a cross-section diagram showing a laser structure of the GaAs infrared semiconductor laser device in a related art.
The GaAs infrared semiconductor laser device is made up of an n-type GaAs substrate 23, an n-type AlGaAs cladding layer 24, an undoped-AlGaAs active layer 25, a first p-type AlGaAs cladding layer 26, an n-type AlGaAs blocking layer 27, a second p-type AlGaAs cladding layer 28, a p-type GaAs contact layer 29, an AuGeNi/Au ohmic electrode 30, and a Ti/Pt/Au ohmic electrode 31.
In the GaAs infrared semiconductor laser device, the high-output operation is realized by controlling light confinement in the waveguide by precisely controlling the alloy ratio of AlGaAs or GaAs using epitaxial regrowth technology. The following layers are sequentially formed on the n-type GaAs substrate 23: the n-type AlGaAs cladding layer 24; the undoped AlGaAs active layer 25; and a portion of the p-type AlGaAs cladding layer that is the first p-type AlGaAs cladding layer 26. On the first p-type AlGaAs cladding layer 26, the n-type AlGaAs blocking layer 27 is formed so as to have a striped opening. On the opening, the rest of the P-type AlGaAs cladding layer that is the second p-type AlGaAs cladding layer 28 and the p-type GaAs contact layer 29 is formed. The AuGeNi/Au ohmic electrode 30 is formed on the rear surface of the n-type GaAs substrate 23. The Ti/Pt/Au ohmic electrode 31 is formed on the p-type GaAs contact layer 29. The light is confined in the striped area because the refractive index of the second p-type AlGaAs cladding layer 28 formed by the regrowth is larger than that of the n-type AlGaAs blocking layer 27. Herein, the light is blocked at the n-type AlGaAs blocking layer so that the GaAs infrared semiconductor laser device has a real refractive index waveguide-type structure with small internal loss. Therefore, low threshold current and high-output operation are realized in the structure (refer to Japanese Laid-Open Patent Publication No. 4-370993 and IO. Imafuji et al., IEEE J. Quantum Electron. QE-29, 1993, p. 1889).
FIG. 2 is a cross-section diagram of the GaN blue-violet semiconductor laser device for explaining the laser structure in the example that the laser structure of the high-output GaAs infrared semiconductor laser device is applied to the GaN blue-violet semiconductor laser device. For example, it is disclosed in Japanese Laid-Open Patent Publications No. 8-97507 and No. 10-027947.
The GaN blue-violet semiconductor laser device is made up of an n-type GaN layer 2, an n-type AlGaN cladding layer 3, an InGaN multiple quantum well active layer 7, a p-type GaN contact layer 11, a Ti/Al/Ni/Au ohmic electrode 12, a Ti/Au pad electrode 14, a Sapphire substrate 16, a Ni/Au electrode 32, a first p-type AlGaN cladding layer 33, an n-type AlGaN blocking layer 34, and a second p-type AlGaN cladding layer 35.
In the GaN blue-violet semiconductor laser device, similar to the GaAs infrared semiconductor laser device shown in FIG. 1, the p-type cladding layer is re-grown on a portion of the p-type cladding layer, and the light confinement in the waveguide is controlled by precisely controlling the alloy ratio of AlGaN. Therefore, a low threshold current operation and a high-output operation can be realized in the GaN blue-violet semiconductor laser device.